The present invention is directed to breathable laminates including at least one microporous film and a continuous elongatable film and a process for making the same. Such materials have a wide variety of uses, especially in the areas of limited use and disposable items.
Films have been traditionally used to provide barrier properties in limited use or disposable items. By limited use or disposable, it is meant that the product and/or component is used only a small number of times or possibly only once before being discarded. Examples of such products include, but are not limited to, surgical and health care related products such as surgical drapes and gowns, disposable work wear such as coveralls and lab coats and personal care absorbent products such as diapers, training pants, incontinence garments, sanitary napkins, bandages, wipes packaging and the like. In personal care absorbent products such as infant diapers and adult incontinence products, films are used as the outer covers with the purpose of preventing body wastes from contaminating the clothing, bedding and other aspects of the surrounding environment of use. In the area of protective apparel including hospital gowns, films are used to prevent cross exchange of microorganisms between the wearer and the patient. In the area of packaging, films are used to allow passage of H.sub.2 O and O.sub.2 but not microorganisms.
Lamination of films have been used to create materials which are both impervious and somewhat cloth-like in appearance and texture. The outer covers on disposable diapers are but one example. In this regard, reference may be had to coassigned U.S. Pat. No. 4,818,600 dated Apr. 4, 1989 and U.S. Pat. No. 4,725,473 dated Feb. 16, 1988. Surgical gowns and drapes are other examples. See, in this regard, coassigned U.S. Pat. No. 4,379,102 dated Apr. 5, 1983.
A primary purpose of the film in such laminations is to provide liquid barrier properties. There is also a need for such laminates to be breathable so that they have the ability to transmit moisture vapor. Apparel made from laminations of these breathable and/or microporous films are more comfortable to wear by reducing the moisture vapor concentration and the consequent skin hydration underneath the apparel item. However, the pore size in breathable films cannot be too large, especially in protective apparel applications, such industrial or medical garments, where chemical liquid penetration presents a contamination risk to the wearer. Moreover, films containing micropores may allow passage of chemical vapors and/or viruses and thereby reduce the effectiveness of the protective apparel.
The conventional process for obtaining a breathable microporous film has been to stretch a thermoplastic film containing filler. Microvoids are created by the filler particles during the stretching process. The film is usually heated prior to these drawing processes to make the film more plastic and malleable. This drawing or stretching also orients the molecular structure within the film which increases its strength and durability in the stretched direction. The molecular orientation caused by stretching is desired to improve durability.
A film can be stretched in the machine-direction or the cross-machine direction. Stretching the film in the cross direction is particularly challenging because forces must be applied to the edges of the film to cause it to elongate. Tenter frames are commonly used. In contrast, stretching the film in the machine direction is relatively easy. It is only necessary to increase the draw, or speed ratio, between two rollers while the film is in the heated and plastic state. There is a durability problem, however, with uni-directionally-stretched films, be it machine direction or cross-direction. Uni-directional stretching causes molecular orientation in the stretched direction. This results in films that are easily torn or split along that dimension. For example, a machine-directionally oriented film has a propensity to split or tear along the machine direction. Also, the tensile characteristics of the film are dramatically increased in the machine direction, but the tensile strength in the cross-direction is significantly inferior to that of the machine direction.
These durability problems with uni-directionally stretched and oriented films are well known. Two approaches are commonly used to obviate the product durability problems resulting from these highly isotropic strength characteristics. The first is to stretch-orient the film in both the machine and cross direction. Films that have been biaxially stretched have more balanced strength properties. The second approach is to combine into a laminate one layer of machine directionally oriented film with one layer of cross-directionally oriented film. This approach is time consuming, size limiting, and costly. There is therefore a need for a uni-directionally-stretched, lightweight breathable film laminate using low-cost materials and processes that provides the laminate with both breathability, barrier and the in-use durability that are desired.
Moreover, precise control of the stretching process is usually required in order to avoid creating holes that are too large since, as previously mentioned, formation of undesirably large pores would lower the hydrohead value of the films to unacceptable low levels and thus cause leaking of liquids, odor causing molecules and microorganisms. There is therefore a need for a flexible breathable laminate and process that provides a laminate with the breathability barrier properties and the fit and comfort that are desired.